Internal combustion engines produce various gaseous emissions, as well as solid particulate matter emissions. In various jurisdictions the mass of certain gases emitted per kilometre travelled is regulated by emissions standards. For example, in the European Union, emissions standards specify maximum masses of carbon monoxide (CO), total hydrocarbons (CxHy), non-methane hydrocarbons and Nitrogen Oxides (NOx) that may be emitted per kilometre by vehicles powered by gasoline engines. Particulate matter emissions from gasoline powered vehicles were previously regulated only in terms of the maximum mass of particulate matter by European emissions standards. However, future standards will set limits on the allowable number of particles emitted by gasoline powered vehicles.
Most modern gasoline powered vehicles are fitted with a “three way” catalytic converter to catalyse reactions that convert the gases whose emissions are regulated to less harmful gases. A typical three way catalytic converter for use in a gasoline powered vehicle catalyses the following reactions:                I. Oxidation of carbon monoxide (CO) to produce carbon dioxide (CO2): 2CO+O2→2CO2         II. Oxidation of unburned hydrocarbons (CxHy) to produce carbon dioxide (CO2) and water (H2O): CxHy+(2x+y/2)/2O2→xCO2+(y/2)H2O        III. Reduction of Nitrogen Oxides (NOx) to produce nitrogen (N2) and oxygen (O2): 2NOx→xO2+N2         
A catalytic converter typically comprises a substrate having a plurality of narrow channels therethrough coated with a washcoat having particles of catalyst suspended therein. The narrow channels provide the converter with a high surface area to volume ratio. Furthermore, once dried, the washcoat typically provides a large surface area, which increases the area of catalyst that the exhaust gas is exposed to. The catalyst is typically a precious metal such as platinum, palladium, rhodium, or a mixture thereof. Washcoats sometimes further comprise an oxygen storage component, such as cerium oxide or zirconia. Such oxygen storage component either releases or takes in oxygen depending on the oxygen concentration in the exhaust gas. This helps to ensure that the oxygen concentration in the catalytic converter is at a level that allows both the oxidation and reduction reactions to take place efficiently.
As gasoline engines typically produce very small amounts of particulate matter, filters to remove particulate matter are not usually fitted to gasoline powered vehicles. However, to reduce particulate number emissions to the levels required by future emissions standards it may be necessary to fit particulate filters to gasoline powered vehicles.
Introduction of particulate filters onto gasoline powered vehicles could potentially cause a number of problems. For example, both particulate filters and catalytic converters cause additional back pressure to the internal combustion engine. This increases pumping losses, thereby decreasing the power that the engine can produce. It also causes a reduction in fuel efficiency, which may also cause an undesirable increase in carbon dioxide (CO2) emissions and increases the likelihood of abnormal combustion events. This increase in back pressure is especially problematic in gasoline engines, because of the high air flow rate required. It is therefore desirable to produce catalytic converters and particulates filters for gasoline powered vehicles that reduce emissions of regulated gases and particulates to acceptable levels without causing a significant increase in back pressure.
Modern catalytic converters also suffer from various problems. For example, catalytic converters only function effectively in an optimum temperature range. Therefore, emissions of carbon monoxide, nitrogen oxides and unburned hydrocarbons may be significantly higher before the catalytic converter is heated up to its operating temperature. It is therefore desirable to produce catalytic converters that heat up faster to reduce emissions generated during catalyst light off.
Furthermore, the concentrations of the different gases emitted by gasoline powered vehicles vary under certain driving conditions. For example, when gasoline powered vehicles are driven at high speed and a high load is placed on the engine, the mass flow rate of exhaust gas is high, and the space velocity over the catalyst increases, which can reduce the conversion efficiency of regulated gaseous emissions such as nitrogen oxides. It is therefore desirable to produce a catalytic converter that increases the efficiency with which nitrogen oxides are converted when the mass flow rate of exhaust gas is high.
The precious metals used in catalytic converters are expensive. Accordingly, it is desirable to reduce the amount of precious metal needed in catalytic converters to produce a desired conversion efficiency.
The present invention aims to at least partially mitigate the problems described above.